The pollution produced by the exhaust from internal combustion engines is increasingly of concern. These pollutants include hydrocarbon, carbon monoxide (CO), nitrogen oxide (NO.sub.x), and particulate emissions. The type and amount of emissions depend, among other things, on the type of engine and fuel system and on operating conditions. For example, diesel engines produce relatively low amounts of CO, but produce significant amounts of particulate matter in the form of soot, that is comprised of carbon, ash, that is comprised of inorganics, and polynuclear aromatic and aliphatic hydrocarbons (PAHs), that are condensed about the carbon nuclei of the soot. 1994 U.S. particulate emissions standards require that diesel engines emit particulates of no more than 0.1 g/BHP/hr. NO.sub.x emissions are also a significant problem for diesel engines.
Porous ceramic and other filters have been used to capture unwanted particulate matter in the form of soot, ash, and PAHs condensed about the carbon nuclei of the soot, which are entrained in the emission stream of diesel engines. The soot is "sticky" and adheres quite readily to the walls defining the pores of the ceramic and other filters. However, after prolonged filtration, the soot so accumulates in the filters as to obstruct the pores. An obstructed filter induces a back pressure in the exhaust line which can affect engine operation and reduce the effective throughput of the filters, necessitating the cleaning or replacement of the filters.
Thermal regeneration to remove the accumulated soot from the filters is known, such as by embedding resistive filaments in the ceramic matrix that oxidize the accumulated soot when energized. However, because hot spots tend to be formed thereby that cause thermal failures in the ceramic, not only is care required to prevent degradation of the filter matrix in the locale of the hot spots, but also degraded filters must be periodically monitored to ensure that they comply with the clean air emission standards. Fine ceramic particles can also be eroded and travel downstream, where they can cause damage to the exhaust system piping or to the engine. Further, the PAHs entrained in the diesel exhaust condense at and around 200.degree. to 400.degree. C. Filters which employ thermal regeneration techniques are generally located at the diesel exhaust manifold close to the engine and typically operate at temperatures well above the boiling point of the PAHs, which makes them generally unsuited to unburned PAH emission control or use in a recirculation line. Moreover, thermally regenerated filters are prone to failure by melting and cracking of the ceramic matrix during the high-temperature regeneration periods.
An alternative to thermal regeneration of the soot filters is aerodynamic regeneration using pulses of compressed air flowing through the trap in a direction opposite to the exhaust. In the aerodynamically regenerated traps, the filter encounters relatively low temperatures, in the range of 200.degree. C. to 300.degree. C., since these traps can be placed at any location in the exhaust pipe, even far from the engine. Moreover, the intermittent pulsing of the regeneration compressed air further cools the filter. An example of an aerodynamically regenerated trap is shown in U.S. Pat. No. 4,875,335, entitled "Apparatus and Method for Treating an Exhaust Gas From a Diesel Engine." In U.S. Pat. No. 5,013,340, entitled "Rotating Diesel Particulate Trap", incorporated herein by reference, particulates are continuously removed by rotating a particulate trap such that, while one sector thereof is exposed to diesel exhaust flowing in one direction, another sector thereof is exposed to a counter flowing stream of high-velocity (high-mass) air provided either by a fan or a compressed air tank.
Early aerodynamically regenerated traps channeled the regeneration air to baghouses, where the soot was retained in fiber bags. The bags were cleaned or replaced as needed. The traps functioned effectively in this configuration, since the large filtration area of the fiber bags offered minimal resistance or back pressure to the flow of the regeneration air through the ceramic filter. However, periodically, the bags must be collected and removed, creating a disposal problem. Thus, particulate trap systems were developed incorporating incinerator sections which burned the particulates in a separate chamber, away from the ceramic filter. By burning the particulates away from the ceramic filter, the filter does not experience elevated temperatures and thermal failures are avoided.
A known incineration system uses a dead-flow cylinder positioned directly below the ceramic filter. A heating element is located at the bottom of the cylinder. If the volume of the dead-flow cylinder is sufficiently large, the momentum of the regeneration air is dissipated in the cylinder and the soot eventually settles on the heater. If the volume of the dead-flow cylinder is small, however, the effectiveness of this system is reduced. The performance of this system is satisfactory if regeneration is performed off line, i.e., while the engine is stopped and no exhaust is flowing through the filter. If regeneration occurs on-line, the cleaning effectiveness of the filter deteriorates with time, probably caused by the re-entrainment of soot in the engine-exhaust stream and re-entry into the ceramic filter. Blocking the exit of the incineration chamber with a fibrous filter has not been found to improve this system, since the filter creates large back pressures, impedes the flow of the regeneration air, and quickly becomes plugged.
Exhaust gas recirculation (EGR) is another known pollution control technique which has been successfully used to reduce NO.sub.x emissions in the exhaust stream from a diesel engine. With EGR, a portion of the exhaust is recirculated back into the engine. The exhaust gas replaces a portion of the combustion air in the engine, resulting in less oxygen available to enter into the reactions, and lowers the temperature at which combustion occurs. A lower concentration of NO.sub.x emissions in the exhaust gas stream results.